Winter+2003+Exam

1) What a system designer leaves out of an architecture is often as important as what is included. Choose one of the operating systems that we discussed in class for which some feature normally considered an integral part of an operating system was not included. Explain the reasons why the designer (or designers) decided to leave this feature out of their design.

-Exokernel leaves out virtual memory, for customization purposes -Pilot does not have protection, it just slows you down -Pilot does not have users, priority scheduling

Problem 4The VAX-11/780 memory-management architecture supported paging, but it did not provide a page-referenced bit. a) Why is it important to have a page-referenced bit when implementing demand paging?

- The reference bit is used to implement LRU replacement

b) How did the authors of the paper Converting a Swap-Based System to do Paging in an Architecture Lacking Page-Referenced Bits surmount the problem of the VAX-11/780 not providing page-referenced bits?

They use a FIFO queue and move a referenced page to the front

c) Briefly explain how a similar technique can be used to provide copy-on-write semantics on shared read/write data spaces.